The invention is directed to a positioning device for an optical disc storage which has a scanning unit, a control unit, a laser light source creating a light beam which is focussed onto the selected information track of an optical storage disc and means provided for coarse positioning the scanning unit and means for fine positioning the scanning unit which means form a regulating loop in cooperation with the detector arrangements which produces track error signals and focal error signals for the scanning unit.
In disc stores, whether a magnetic or optical type, the characteristics of the positioning device are always essentially important. Technological advances have allowed an increase in storage capacity and here an increase in track density is of particular interest, but, at the same time, every effort has been made to reduce the average access time in order to increase the efficiency of the disc store.
Therefore, there has been no lack of attempts to fulfill the inherently contradictory requirements of a short access time and a high track density. In a positioning system, a moving mass normally occurs in the form of a scanning unit which moves fundamentally radially to the storage disc. In particular, in magnetic disc stores, technical advances, in particular relating to the magnetic heads, have permitted a substantial reduction in the mass of the scanning unit which must be necessarily moved and has thus, also permitted a high speed positioning system. However, because in optical imaging systems it is more difficult to reduce the moving mass of the scanning unit, different technological limits exist.
A relatively high access speed, for example, a high speed radial movement of the positioning device in the case of track changes, can also be more easily achieved in a known manner with positioning devices of a multi-stage design. Here, a coarse positioning of the scanning unit, whether it is a magnetic right head or an optical scanner, is carried out to a track area as close as possible to the selected data track or to the selected data track itself. This coarse positioning is then followed by a fine positioning, which either adjusts the scanning unit to the selected track and/or holds this track position. This division of the positioning procedure into two stages, which comprise a high translation speed in the first stage and a precise fine positioning in the second stage on average permits an optimized access time under the existing limiting conditions using two control systems.
Optical data stores are generally used only for filing purposes so that here the problem of average access time during the continuous read-out of large quantities of data is not of such a decisive importance. If, however, optical data stores are to compete with magnetic disc stores, for example, optical stores are to be able to be used for operating modes involving frequent track changes, very short access times must be feasible in these data stores. Hitherto, this problem has not been satisfactorily solved, in particular, due to the inertia of the positioning device which in turn in governed by the mass of the scanning unit which is to be moved in the radial direction.